If you are talking km/hr that sounds credible. Motor Trend got 4.7 versus a BMW estimate of 5.1 for the coupe in 2008. The stuff I see on-line is 5.3 for a convertible. 4 tenths less would put you at 4.9.

I think it takes a good driver to beat the factory numbers, however. It may need to be good ambient conditions etc. too.